1. Field of the Invention
This invention relates to a dental implant system. More particularly, this invention relates to a dental implant system having an implant body and an abutment, wherein the implant body and abutment format least a first anti-rotational connection by frictional engagement of mating tapered surfaces.
2. Background of the Invention
Two-part endosseous dental implant systems for insertion in a wholly or partially edentulous region of the jawbone of a patient are known in the art. The implant systems may be completely embedded in a jawbone of a patient. Typically, a protective cover screw is attached to the top of the implant. The implant is then covered with mucosal tissue. Alternatively, the implants or a protective component affixed to the implant may protrude through the oral mucosa at the time of placement of the implant into the jawbone. Typically, the implants are permitted to remain in place while new bone grows around the implant. Once the implant has become firmly anchored in bone, the mucosal tissue must be reopened if the implant is covered. The protective component is then removed and an abutment or post is connected to the implant using a screw. A prosthesis can then be connected to the abutment or post.
Many two-part implant systems have an external, hexagonal projection, sometimes called a male hex, which projects upwardly from the top end of the implant. A shoulder surrounds the base of the male hex. An abutment or post having an outer diameter that substantially matches the outer diameter of the implant is seated on the male hex to form a substantially sealed connection. Some implants have an externally-threaded sidewall portion that can be screwed into an opening formed in the bone after bone tissue has been removed from the jawbone. Examples of such an implant may be found in U.K. Patent No. 1,291,470 or in U.S. Pat. No. 4,713,004. With implant systems of this kind, the male hex projection at the top of the implant is designed to engage an inserting device, e.g. a wrench, that is used to insert the implant in the jawbone.
Another kind of two-part, endosseous dental implant system with an external male hex is a cylindrical implant having a non-threaded, external body portion. These implants are pushed into an opening formed in bone tissue. An example of this type of implant is a BIO-VENT(copyright) implant available from Core-Vent Corporation, 15821 Ventura Boulevard, Suite 420 Encino, Calif. 91436.
In implants having external male heads, the male head is used to attach the implant to an abutment or post having a matching female hex-shaped cavity that receives and engages the male hex projection. Such male hex heads and female hex cavities are sometimes referred to as coupling surfaces. Typical implant systems have external male hexes and mating internal female hex cavities with walls of the hexagonal head and the hex-shaped cavity of the abutment being perpendicular to a longitudinal axis of the abutment and parallel to one another.
With such implant systems, the male hex of the implant is smaller in diameter than the diameter of the hex-shaped cavity of the abutment to permit the male hex to fit inside the female cavity. The difference in diameter is sufficiently large to allow for manufacturing variations while still allowing the coupling surfaces of the abutment to seat fully on the shoulder of the implant. Seating the coupling surfaces on the shoulder of the implant creates a sealed outer margin between abutment and implant. However, this leaves space between the coupling surfaces of the male and female hexes.
Within the hex head region, and extending into the implant itself, there is in such implants a threaded hole for receiving an attachment screw of a mating abutment. The abutment typically has an interior abutment passage centered on its hex cavity. When attaching the abutment to the implant, the screw is inserted through the abutment passage and is screwed into the threaded implant hole. Tightening the screw tightens the abutment against the implant. When the screw is tightened until the external hex of the implant mates with the matching female hex cavity in the abutment, the system is secured against axial displacement of the abutment from the implant.
The seating of the external hex of the implant within the female hex cavity of the abutment, where both the external hex and the internal hex cavity have parallel walls, results in the full seating of the abutment onto the shoulder surrounding the external male hex of the implant. However, according to reported studies, the seating of the external hex of the implant within the female hex cavity of the abutment of existing implant systems fails to completely prevent rotational displacement of the implant with respect to the abutment.
For example, a scientific study presented by Dr. Paul Binion at the Academy of Osseointegration meeting in San Diego, Calif. in Mar. 1993, documented that the coupling surfaces of commercially-available implants allow four to five degrees of rotation between the abutment and the implant. Dr. Binion later reported that certain implant/abutment assemblies exhibit up to nine degrees of rotation between the implant and the abutment. The relative rotation of the abutment and implant result in an attachment that is unstable. Lateral forces from biting are transmitted to the screw joining the abutment to the implant rather than the coupling surfaces of the external hex projection on the implant and the internal hex cavity in the abutment. As a result, the screw that joins the implant to the abutment may break or loosen. Rotational instability may also adversely affect the accuracy of transfer procedures needed for the indirect fabrication of a final prosthetic restoration on such implant/abutment assemblies.
Attempts have been made to remedy the problem of rotational instability in implant/abutment assemblies. For example, U.S. Pat. No. 4,547,157 discloses an implant having a conical projection for mating with an abutment having a matching cavity. A small degree of taper of the two surfaces results in a friction fit between the parts that tends to maintain the connection. These systems do not use a screw that passes through the abutment to lock the abutment to the implant. The tapered, cylindrical coupling surface makes direct contact and fully seats on the mating cavity in the implant, which results in a good connection. However, a drawback with this type of connection is that a ledge is formed as the outer walls of the internal cavity fit over the conical projection of the implant. This ledge can trap food particles and irritate gum tissues. Moreover, it is necessary to use a hammering action to seat the abutment onto the implant, which is uncomfortable for a patient. Further, it is not possible to quantify the force of hammering, which varies greatly from one practitioner to the other.
Other implants exist that have an internal taper connection. One example is the ITI(copyright) Dental Implant System available from Straumann Holding AG, CH-4437 Waldenburg, which has a very wide implant head to accommodate the abutment. Therefore, the ITI(copyright) dental implant is not suitable for narrow spaces. In addition, the marginal area of the restoration is cemented directly onto the implant. As a result, the top of the implant must remain exposed after healing of the soft tissues. A disadvantage of this arrangement is that it is not possible to modify the marginal area, which leaves a visible unaesthetic silver margin around the restoration.
There is, therefore, a need for improvements in dental implant systems, particularly, endosseous dental implant systems which overcome the above and other disadvantages.
The present invention is directed to an endosseous dental implant system that includes at least two parts: a first part called a implant body, and a second part called an abutment, post or insert. The implant body may have a threaded external sidewall surface or a non-threaded external sidewall surface, and the implants themselves may be generally cylindrical or tapered in shape. The external sidewall surface may also have one or more longitudinally extending grooves.
A part or all of the external surfaces of the implant system may be treated by applying a coating consisting of hydroxyl apatite or titanium plasma spray. Alternatively, part or all of the external surfaces may be roughened by blasting or acid etching or a combination of the above-mentioned methods.
A tapered cylindrical surface is provided at the top end of the implant body for engaging and interlocking anti-rotationally with a matching tapered cavity inside the abutment. The anti-rotational connection is formed when the abutment is fully seated and fastened to the implant body by means of screw or fastener. On top of the implant, there may be an additional projection, preferably a multi-sided projection. In the preferred embodiment, the projection consists of multiple sides that are parallel to the longitudinal axis of the implant body. The optional projection forms a second anti-rotational connection with a corresponding internal cavity in the abutment.